Merge branch 'rr/push-head'

"git push $there HEAD:branch" did not resolve HEAD early enough, so
it was easy to flip it around while push is still going on and push
out a branch that the user did not originally intended when the
command was started.

* rr/push-head:
  push: make push.default = current use resolved HEAD
  push: fail early with detached HEAD and current
  push: factor out the detached HEAD error message